Detect pre-reqs and emit diagnostics

[Marc Gravell]

• <LangVersion>11</LangVersion> (or higher)
• <Features>interceptors</Features>

2 separate diagnostics; also, do not emit anything if langver is too low (currently: we do)

note: it should only emit these if at least one Dapper call is detected - suggest adding a new flag that is AotNotEnabled, and all (non-zero) are AotNotEnabled: do the thing; perhaps implement as optional , flags = None on the simple ctor, and this flags = flags | DoNotGenerate